1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a data transmission/reception apparatus and method for improving performance in a mobile communication system using a space-time trellis code (STTC).
2. Description of the Related Art
With the rapid progress of mobile communication systems and the increasing amount of data being serviced in the mobile communication systems, 3rd generation (3G) mobile communication systems for transmitting data at a high rate have been developed. Such 3G mobile communication system adopt asynchronous Wideband-Code Division Multiple Access (W-CDMA) technology in Europe and synchronous Code Division Multiple Access-2000 (CDMA-2000) technology in North America as radio access standards. Commonly, in the mobile communication systems, mobile stations (MSs) communicate with each other via a base station (BS).
However, in the existing mobile communication systems, during high-speed data transmission, a received signal undergoes phase distortion due to a natural fading phenomenon occurring on a radio channel. The fading reduces the amplitude of a received signal by several dB up to several tens of dB. This fading also distorts the phase of a received signal, and if not compensated for during data demodulation, causes information errors in the data transmitted by a transmission side, which in turn causing a deterioration in the quality of a mobile communication service. In order to transmit high-rate data without deterioration of the service quality, the mobile communication system must overcome the fading, and in order to overcome the fading, several diversity technologies are used.
A CDMA system utilizes a Rake receiver that performs diversity reception using a delay spread of a channel. Although the Rake receiver employs reception diversity for receiving a multipath signal, a Rake receiver employing diversity technology using the delay spread does not work when the delay spread is less than a predetermined value. In a Doppler spread channel, time diversity technology using interleaving and coding is used to compensate for fading, but the time diversity technology is not efficient in a low-speed Doppler spread channel.
In order to cope with the fading, space diversity technology is used in a channel with a low delay spread such as an indoor channel, and a channel with a low Doppler spread such as a pedestrian channel. The space diversity uses two or more transmission and reception antennas. That is, when a signal transmitted via one transmission antenna decreases in amplitude due to fading, the space diversity technology receives a signal transmitted via the other transmission antenna. The space diversity can be classified into reception antenna diversity technology using reception antennas and transmission diversity technology using transmission antennas. However, because the reception antenna diversity technology is applied to a mobile station, it is difficult to install a plurality of reception antennas the mobile station in light of its small size and increased installation expenses. It is recommendable to use the transmission diversity technology that installs a plurality of transmission antennas in a base station.
Particularly, a 4th generation (4G) mobile communication system anticipates a data transfer rate of 10 Mbps to 150 Mbps, and requires a bit error rate (BER) of 10−3 for voice and a BER of 10−6 for data. The STTC is combination of multiple antennas and channel coding technology, and is a technology that contributes to remarkable improvement in a data rate and reliability in a radio multi-input/multi-output (MIMO) channel. The STTC can obtain space-time diversity gain by extending the space-time dimension of a transmission signal from a transmitter. In addition, the STTC can obtain coding gain without additional bandwidth, contributing to great improvement in channel capacity.
It is preferable to use the STTC when applying the transmission diversity technology, and when a plurality of transmission antennas are used, the use of the STTC obtains coding gain by amplifying transmission power, together with diversity gain corresponding to a reduction in channel gain occurring due to a fading channel.
A method for transmitting a signal using the STTC is disclosed in a reference entitled “Space Time Codes For High Data Rate Wireless Communication: Performance Criterion And Code Construction”, by Vahid Tarokh, N. Seshadri, and A. Calderbank, IEEE Trans. on Info. Theory, pp. 744-765, Vol. 44, No. 2, March 1998, the contents of which are incorporated herein by reference.
With reference to FIG. 1, a description will now be made of a structure of a transmitter using a serially-concatenated space-time code encoding apparatus.
FIG. 1 is a block diagram illustrating a structure of a transmitter using a serially-concatenated space-time code encoding apparatus. Referring to FIG. 1, a serially-concatenated space-time code (SC-STC) encoding apparatus has a serial concatenation structure in which an interleaver 110 is interposed between an outer encoder 100 and an inner encoder 120. That is, the SC-STC encoding apparatus has two encoders serially concatenated with an interleaver interposed therebetween.
The outer encoder 100 is a general channel encoder, for which, for example, a convolutional encoder, a turbo encoder, and a low density parity check (LDPC) encoder can be used. Because the channel encoder is not related to the present invention, a detailed description thereof will be omitted. Although a variety of interleavers can be used for the interleaver 110, it will be assumed herein that a random interleaver is used, for the convenience of explanation.
The inner encoder 120 uses the STTC code in order to obtain a space-time diversity effect in a multi-antenna system. Therefore, a transmission data sequence ‘u’ is channel-encoded through the outer encoder 100, and an encoded sequence ‘a’ interleaved through the interleaver 110 is output as symbols s1 and s2 through the inner encoder 120, or an STTC encoder. The output symbol s1 is transmitted via a first transmission antenna TX1, and the output symbol s2 is transmitted via a second transmission antenna TX2.
Generally, a recursive STTC (R-STTC) encoder for delaying, feeding back output data, and calculating the output data with a next signal is used as the STTC encoder 120.
A structure of the R-STTC encoder will now be described in detail with reference to FIG. 2.
FIG. 2 is a block diagram illustrating a structure of a conventional quadrature phase shift keying (QPSK) modulation-based R-STTC encoder used as an inner encoder.
Referring to FIG. 2, the QPSK modulation-based R-STTC encoder has a structure in which two recursive convolutional codes are connected in parallel, and is comprised of, for example, two binary adders 201 and 205 and two delays 203 and 207. The R-STTC encoder illustrated in FIG. 2 outputs 4 STTC-encoded symbols S11, S21, S12 and S22 for 2 channel-encoded input bits a1 and a2 by QPSK modulation. The R-STTC encoder has a structure comprised of two recursive convolutional codes. The number of the recursive convolutional codes can be increased according to a change in a modulation method. The number of the binary adders and delays can also be changed depending on how the recursive convolutional codes are formed. For example, an 8-ary phase shift keying (8PSK) modulation-based R-STTC encoder is comprised of 3 binary adders and 3 delays, and a description thereof will be made with reference to FIG. 3. In the same manner, the R-STTC encoder can also be implemented based on 16-ary quadrature amplitude modulation (16QAM), 64QAM, and 128QAM.
The QPSK modulation-based R-STTC encoder illustrated in FIG. 2 outputs 4 STTC-encoded symbols through recursive convolutional encoders 200 and 210 in order to transmit channel-encoded input signals a1 and a2 via a first transmission antenna TX1 and a second transmission antenna TX2 while obtaining a space-time diversity effect. Here, each of the recursive convolutional encoders is an encoder for generating compound codes with a code rate of ½, and performs QPSK modulation so that the two compound codes are parallel-concatenated.
The channel-encoded input signals a1 and a2 are channel-encoded symbols, continuously output through the channel encoder (i.e. outer encoder). The symbols continuously output through the channel encoder are sequentially mapped to input terminals of the R-STTC encoder as the input signals a1 and a2.
The input signal a1 is output as a symbol S11 through the first binary adder 201 and the first delay 203, and then transmitted via the first transmission antenna TX1, and the symbol S11 is fed back to the first binary adder 201 where it is added to a next input signal. An output symbol S12 calculated by adding the current input signal to a previous signal in the first binary adder 201 is transmitted via the second transmission antenna TX2.
The input signal a2 is output as a symbol S21 through the second binary adder 205 and the second delay 207, and then transmitted via the first transmission antenna TX1, and the symbol S21 is fed back to the second binary adder 205 where it is added to a next input signal. An output symbol S22 calculated by adding the current input signal to a previous signal in the second binary adder 205 is transmitted via the second transmission antenna TX2.
The conventional R-STTC encoder adds channel-encoded input signals to their previous output signals, and then transmits output symbols delayed through the delays via one transmission antenna (i.e. the first transmission antenna). The R-STTC encoder transmits output symbols added through the binary adders via another transmission antenna (i.e. second transmission antenna) before being delayed by the delays.
When output symbols of the R-STTC encoder shown in FIG. 2 are expressed in the form of a matrix, the output matrix G(D) can be expressed as
                                                                        G                ⁡                                  (                  D                  )                                            =                              (                                                      G                    1                                    ,                                      G                    2                                                  )                                                                                        =                              (                                                                                                    D                                                  (                                                      1                            +                            D                                                    )                                                                                                            0                                                                                      1                                                  (                                                      1                            +                            D                                                    )                                                                                                            0                                                                                                  0                                                                                      D                                                  (                                                      1                            +                            D                                                    )                                                                                                            0                                                                                      1                                                  (                                                      1                            +                            D                                                    )                                                                                                                    )                                                                        (        1        )            
In Equation (1), G1 denotes a symbol output via the first transmission antenna TX1, and G2 denotes a symbol output via the second transmission antenna TX2. D denotes a delay. In the matrix, each row represents a recursive convolutional code which is a compound code. A first row represents the first recursive convolutional encoder 200 and a second row represents the second recursive convolutional encoder 210. Each column represents an output symbol. The first two columns (i.e. first and second columns) represent output symbols of the R-STTC encoder, transmitted via the first transmission antenna, and the last two columns (i.e. third and fourth columns) represent output symbols of the R-STTC encoder, transmitted via the second transmission antenna. It can be noted that the conventional R-STTC encoder has a parallel concatenation structure of two non-systematic convolutional encoders.
So far, the structure of the QPSK-based R-STTC encoder has been described. Next, an 8PSK-based R-STTC encoder will be described with reference to FIG. 3.
FIG. 3 is a block diagram illustrating a structure of a conventional 8PSK modulation-based R-STTC encoder used as an inner encoder. Referring to FIG. 3, the R-STTC encoder is comprised of 3 binary adders 301, 305 and 309, and 3 delays 303, 307 and 311.
The R-STTC encoder illustrated in FIG. 3 outputs 6 STTC-encoded symbols S11, S21, S31, S12, S22 and S32 for 3 channel-encoded input bits a1 a2 and a3 by 8PSK modulation. Therefore, the R-STTC encoder has a structure comprised of three recursive convolutional codes.
Each of the recursive convolutional encoders is an encoder for generating compound codes with a code rate ½, and performs 8PSK modulation so that the three compound codes are parallel-concatenated.
The 8PSK modulation-based R-STTC encoder illustrated in FIG. 3 outputs 6 STTC-encoded symbols through recursive convolutional encoders 300, 310 and 320 in order to transmit channel-encoded input signals a1, a2 and a3 via a first transmission antenna TX1 and a second transmission antenna TX2 while obtaining a space-time diversity effect.
The input signal a1 is output as a symbol S11 through the first binary adder 301 and the first delay 303, and then transmitted via the first transmission antenna TX1, and the symbol S11 is fed back to the first binary adder 301 where it is added to a next input signal. An output symbol S12 calculated by adding the current input signal to a previous signal in the first binary adder 301 is transmitted via the second transmission antenna TX2.
The input signal a2 is output as a symbol S21 through the second binary adder 305 and the second delay 307, and then transmitted via the first transmission antenna TX1, and the symbol S21 is fed back to the second binary adder 305 where it is added to a next input signal. An output symbol S22 calculated by adding the current input signal to a previous signal in the second binary adder 305 is transmitted via the second transmission antenna TX2.
The input signal a3 is output as a symbol S31 through the third binary adder 309 and the third delay 311, and then transmitted via the first transmission antenna TX1, and the symbol S31 is fed back to the third binary adder 309 where it is added to a next input signal. An output symbol S32 calculated by adding the current input signal to a previous signal in the third binary adder 309 is transmitted via the second transmission antenna TX2.
The 8PSK-based R-STTC encoder of FIG. 3, like the QPSK-based R-STTC encoder of FIG. 2, adds channel-encoded input signals to their previous output signals, and then transmits output symbols delayed through the delays via one transmission antenna (i.e., the first transmission antenna). The R-STTC encoder also transmits output symbols added through the binary adders via another transmission antenna (i.e., second transmission antenna) before being delayed by the delays.
When output symbols of the R-STTC encoder shown in FIG. 3 are expressed in the form of a matrix, the output matrix G(D) can be expressed as
                                                                        G                ⁡                                  (                  D                  )                                            =                              (                                                      G                    1                                    ,                                      G                    2                                                  )                                                                                        =                              (                                                                                                    D                                                  (                                                      1                            +                            D                                                    )                                                                                                            0                                                              0                                                                                      1                                                  (                                                      1                            +                            D                                                    )                                                                                                            0                                                              0                                                                                                  0                                                                                      D                                                  (                                                      1                            +                            D                                                    )                                                                                                            0                                                              0                                                                                      1                                                  (                                                      1                            +                            D                                                    )                                                                                                            0                                                                                                  0                                                              0                                                                                      D                                                  (                                                      1                            +                            D                                                    )                                                                                                            0                                                              0                                                                                      1                                                  (                                                      1                            +                            D                                                    )                                                                                                                    )                                                                        (        2        )            
In Equation (2), G1 denotes a symbol output via the first transmission antenna TX1, and G2 denotes a symbol output via the second transmission antenna TX2. D denotes a delayer. In the matrix, each row represents a recursive convolutional code which is a compound code. A first row represents the first recursive convolutional encoder 300, a second row represents the second recursive convolutional encoder 310, and a third row represents the third recursive convolutional encoder 320. In addition, each column represents an output symbol. The first three columns (i.e., first, second and third columns) represent output symbols of the R-STTC encoder, transmitted via the first transmission antenna, and the last three columns (i.e., fourth, fifth and sixth columns) represent output symbols of the R-STTC encoder, transmitted via the second transmission antenna. It can be noted that the conventional R-STTC encoder has a parallel concatenation structure of three non-systematic convolutional encoders.
The conventional STTC code described above is capable of obtaining both coding gain and diversity gain, because it is a code designed by simultaneously taking channel coding, a modulation technique and use of multiple antennas into account. A method for forming SC-STC by serially concatenating an error correction code as the outer encoder shown in FIG. 1 with the STTC encoder is being actively studied.
However, in the future radio communication system requiring the increasing channel capacity and data rate in order to implement the 3G or 4G mobile communication system, performance improvement of the SC-STC is required. Although a study on a channel encoder as an outer encoder constituting the SC-STC is being actively perused, the studies on using an STTC encoder as the inner encoder is insufficient.
Furthermore, in the next generation mobile communication system aiming at transmitting data via multiple antennas with improved performance, improving performance for the STTC encoder is necessarily required.